To avoid inadvertent triggering of a safety device it has been proposed to utilise two units, namely an arming unit and a crash sensing unit, to produce, respectively, an arming signal and a crash signal, the safety device being triggered only when the arming signal and crash signal are present simultaneously. An arrangement of this type is intended to reduce the risk that a single mechanical or electrical error might lead to an inadvertent triggering of the safety device.
The arming unit may comprise a simple acceleration sensor which generates the arming signal if the acceleration exceeds a predetermined level. Alternatively the arming unit may include one or more sensors and a processing unit which performs a predetermined algorithm, the arming algorithm, on the signal or signals received from one or more sensors to provide the appropriate output.
The crash sensing unit may again comprise a simple acceleration sensor which generates a signal if the acceleration exceeds a predetermined level, but again the crash sensing unit may comprise one or more sensors associated with a processor which is configured to perform an algorithm, the crash algorithm, on the signal or signals received from one or more sensors to provide the appropriate output.
The crash sensing unit typically includes a sensor which is directionally sensitive, the sensitive direction of the accelerometer being aligned with the crash direction which is to be sensed by the sensor. The crash algorithm may be determined by whether or not the acceleration sensed by the accelerometer, acceleration “a” or its integrated value, Δv exceed a predetermined threshold.
It has been proposed to use the signal from a proximity sensor configured to provide an output related to the distance to an object and/or the relative speed between the object and the sensor, such as a doppler radar, to improve the ability to discriminate between different potential crashes, so as, for example, to discriminate between a potentially dangerous crash and a potentially not dangerous crash. If a doppler radar is utilised, the output represents the relative speed vrel between the sensor and a nearby obstacle.
It has been suggested, for example in U.S. Pat. No. 5,835,007, to utilise an anticipatory crash sensor of the radar type in conjunction with an accelerometer, with the threshold value of acceleration being calculated in dependence upon the signal from the anticipatory crash sensor.
Whilst a sophisticated arrangement incorporating a high number of sensors of different types may provide great accuracy in triggering a safety device at an appropriate instant, the provision of a large number of sensors leads to a relatively high cost.
The present invention seeks to provide an improved arrangement for triggering a vehicle safety device.